Studies on molecular structures and oligosaccharide-binding specificities of algal lectins

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The research results showed that the lectins from twelve algal species examined have the highly binding specificity to some definite structures of oligosaccharides such as high-mannose type, complex type, or both type of N-glycans, forssman antigen, or sialyl Lewis X, respectively. The high-mannose or complex type N-glycan specific lectins were further divided into several groups by the difference of the branched structures recognized. The lectins thus recognize the branched moiety of the N-glycans. Interestingly, however, the reducing terminal N-acetylchitobiose of the N-glycans was essential for the binding of these lectins to the glycans, because they did not bind to any of the branched oligosaccharides themselves. Thus these algal lectins had the rigid binding specificity for some oligosaccharide structures.Among the high-mannose type N-glycan specific lectins, the complete primary structures of the lectins from the red alga Eucheuma serra and the blue-green alga Oscillatoria agard … More hii were determined. Both lectins had the very similar sequences to each other including the tandem repeat structures of homologous sequences of the N-terminal 67 amino acids. The number of the repeats was different between E.serra lectin (267 amino acids of 4 repeats) and O.agardhii lectin (132 amino acids of 2 repeats). On the other hand, the number of the repeats was well agreement with that of oligosaccharide-binding sites per a monomeric molecule for both lectins. The results indicate that a repeating unit corresponds to an oligosaccharide-recognition domain and the monomeric lectins agglutinate cells by having the multiple carbohydrate-binding sites on each single polypeptide chain. In homology search, surprisingly, the sequences of both algal lectins showed the very high similarity with that of a bacterium Myxococcus xanthus agglutinin including the four tandem repeats of the N-terminal 67 amino acids. Thus it found that a new lectin family having structural similarity was present among the lower organisms of the red alga, the blue-green alga (cyanobacterium) and the bacterium. This is the first example that the structural similarity of lectin molecules was found between prokaryote and eukaryote organisms. Less